When a WDM optical network has to be dimensioned in order to address a given traffic matrix, multilayer network design optimization consists in taking into account jointly the constraints of the diverse layers composing the WDM optical network, e.g. an optical transport layer and an electrical client layer. Multilayer network design optimization aims at reducing the cost of the network.
The network parameters that are available for optimization typically include:                Path selection for each traffic demand, as there are many possible paths from the source to the destination. Besides, each traffic demand may have a different class of service.        Intermediate grooming points, i.e. points of termination of the optical layer at intermediate nodes to reduce a total number of WDM interfaces by making efficient use of wavelength channels.        Physical layer parameters (e.g. channel capacity) and constraints (e.g. wavelength continuity and physical transmission reach).        
A network planning method that would seek an optimal solution, i.e. the one providing the lower network cost for all parameters in a real network would be very complex and time consuming.